Garmin 400C Owner's Manual page 20

I F 400c
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Because water is necessary to carry the sounder's sonar signal, the
transducer must be in the water to work properly. You cannot get a
depth or distance reading when out of the water.
When you place your boat in the water check for leaks around
any screw holes that were added below the water line. DO NOT
leave your boat in the water for an extended period of time without
checking for leaks.
To test the transom mount transducer installation:
1. Begin testing the installation at a slow speed. If the sonar
appears to be working properly, gradually increase the
boat's speed while observing the sonar's operation. If the
sonar signal is suddenly lost or the bottom return is severely
degraded, note the speed at which this occurs.
2. Return the boat to the speed at which the signal was lost. Make
moderate turns in both directions, and see if the signal improves.
4
3. If the signal strength improves while turning, adjust the
transducer so that it extends another 1/8" below the transom
of the boat. It might take several adjustments to eliminate the
degradation.
4. If the signal does not improve, you might have to move the
transducer to a different location.
NOtE: When adjusting the depth of the transducer, make the
adjustments in small increments. Placing the transducer too
deep can adversely affect the boat's performance and put the
transducer at greater risk of striking underwater objects.
Fishfinder 400C Owner's Manual